Date of this Version
Freshwater Biology (2000) 43, 107-119.
1. We examined spatial patterns in population characteristics (density, biomass, mean body length) and physiological condition (lipid content, length-weight) of the amphipod Diporeia spp. in Lake Michigan by collecting samples at up to 85 sites in late summer 1994 and 1995. Variables were examined relative to water depth and three lake regions: south, central and north. Most major river systems are found in the south, and this region is more nutrient-enriched compared to the north.
2. Over all sites, mean density was 5240 m-2, biomass was 4.1 g dry wt m-2, and mean body length was 5 mm. While maximum densities were related to depth, with a peak at 30-70 m, greatest densities occurred on the west side of the lake, and low densities were found in the south-east, north-east and lower Green Bay. High densities in the west probably resulted from upwelling, and reduced densities in the south-east may reflect food competition with Dreissena polymorpha (zebra mussel).
3. Lipid content, weight per unit length, and mean length declined with increased water depth, but depth-related trends were most evident in the south. Overall, mean lipid content and weight per unit length were significantly lower in the south (16.6% dry wt, 0.59 mg at 5 mm body length) compared to the north (23.7% dry wt, 0.78 mg at 5 mm body length). These regional differences may have resulted from greater diatom availability in the north and competition from D. polymorpha in the south. Triacylglycerols and phospholipids were the dominant lipid classes in all three regions. Although the mean proportion of triacylglycerols, the energy-storage lipid, was lower in the south than in the north, regional differences in proportions of lipid classes were not significant.
4. Mean lipid content and weight per unit length of Diporeia in the south were lower than values found in the late 1980s prior to the establishment of Dreissena. Mean lipid content of mature individuals is now at levels considered a minimum for successful reproduction.